The core of nonlinear laser ultrasonic crack detection technology involves the use of a mechanical/photo-thermal loading method to induce loading on cracks, thereby enabling periodic opening and closing of these cracks. The crack closure mechanism has rarely been investigated. To address this issue, in the heated laser irradiation area, the time domain laser ultrasonic method is used to study the closing response of real micro-cracks. The local plastic/elastic microscopic changes induced by the crack closure are also investigated. Analysis of the changes in the peak-peak value and arrival time of each ultrasonic mode in different crack closing states yield pertinent information about the crack closure process. The results reveal that the change process of the signal peak-peak value and arrival time is related to the time of crack closure, and the sequence of crack closure corresponds to the crack width.